Recent observations provide strong evidence that the pH-dependent monovalen anion subfraction of bilirubin mediates tissue uptake and toxicity of bilirubin. The molecular configuration of the mono-anion would be expected to confer surfactant and possibly protonophore properties to bilirubin whic may account for much of its biological behavior. This project examines the effects of pH and bilirubin concentrations on bilirubin-solvent behavior, bilirubin-membrane interaction, and toxicity. In vitro studies will document the influence of bilirubin on surface tension in aqueous solutions and investigate determinants of water/non-aqueous solvent partitioning. Th role of pH-dependent proton translocating and surfactant properties of bilirubin on membrane function will be assessed by evaluating lipid bilayer and synaptosomal plasma membrane permeability to protons and cations, and the ability of bilirubin to alter membrane proton function (reconstituted nicotinic acetycholine receptors). The influence of membrane lipid composition on susceptibility to bilirubin toxicity will be investigated by comparing the behavior of vesicles of varying lipid composition and synaptsomes prepared from adult and newborn rats and Torpedo Californica. The effect of pH on bilirubin toxicity in vivo will be studied using the newborn monkey model (M. Mulatta). Thresholds for neurotoxicity with respect to serum total bilirubin, unbound bilirubin, and unbound monovalent anion concentrations, dissociation rate constant of the bilirubin-albumin complex, and regional brain bilirubin content will be assessed by measuring amplitude changes in the brainstem auditory evoked response in newborn monkeys of differing gestational age (145-165). We hypothesize that early bilirubin toxicity may be reversed by increasing brain pH, thus promoting dissociation of bilirubin-membrane complexes.